1. Field of the Invention
The present invention relates to a charging and discharging control circuit that detects a voltage or abnormality of a secondary battery and a battery device having the charging and discharging control circuit, and more particularly, to a charging and discharging control circuit that can perform a control with a single charging and discharging control MOSFET and a battery device having the charging and discharging control circuit.
2. Background Art
FIG. 3 is a circuit diagram of a battery device having a charging and discharging control circuit according the related art. In the battery device having the charging and discharging control circuit according to the related art, an enhancement type N-channel MOSFET 306 that can bi-directionally intercept electric conduction is connected in series to a negative electrode side of a secondary battery 101. A charging circuit or a load is connected between a V+ terminal 120 and a V− terminal 121 and a charging and discharging current is supplied to or discharged from the secondary battery 101 via the terminals. A control circuit 102 detects the voltages of the secondary battery 101 and the enhancement type N-channel MOSFET 306 and controls the turning-on and turning-off of switches 301, 304, and 305 depending on the detected values. In the enhancement type N-channel MOSFET 306, the drain terminal and the source terminal can bi-directionally conduct electricity to each other when the potential of the gate terminal is equal to or greater than a positive threshold voltage, and the electric conduction between the drain terminal and the source terminal is intercepted when the potential of the gate terminal is less than the threshold voltage.
A charging inhibited state will be described below. When a charger is connected between the V+ terminal 120 and the V− terminal 121, the voltage Vds between the drain terminal and the source terminal of the enhancement type N-channel MOSFET 306 has a positive value. The control circuit 102 detects that Vds is positive, turns on the switch 301, and turns off the switches 304 and 305. Accordingly, the gate terminal of the enhancement type N-channel MOSFET 306 has a potential higher by the voltage of the secondary battery 101 than the source terminal and the enhancement type N-channel MOSFET 306 is switched to an electrically-connected state.
When the secondary battery 101 is charged and the battery voltage reaches a set upper limit value, the control circuit 102 turns off the switch 301 and turns on the switches 304 and 305. Then, the gate terminal of the enhancement type N-channel MOSFET 306 has the same potential as the source terminal and the enhancement type N-channel MOSFET 306 is switched to an OFF state. As a result, a charging current is intercepted to prevent the secondary battery 101 from being overcharged. At this time, a diode 302 is reversely biased to prevent a current from flowing through the switch 304 and the switch 305.
When the charging current is intercepted, a voltage drop due to internal resistance disappears and the voltage of the secondary battery 101 decreases. In order to prevent charging from being restarted due to the decrease in voltage, the charging inhibited state only has to be maintained until the secondary battery 101 is discharged to a certain extent and the voltage thereof becomes equal to or less than a set value after the charging is inhibited. When a load is connected between the V+ terminal 120 and the V− terminal 121 in the charging inhibited state, Vds is switched from positive to negative. The control circuit 102 only has to control the switches 301, 304, and 305 so as to discharge the secondary battery when Vds is negative and to intercept the charging current when Vds is positive.
In the above description, both switches 304 and 305 are turned on at the time of stopping the charging. However, even when the switch 304 is turned off, the charging can be similarly stopped. Since the switch 305 is turned on regardless of the turning-on and turning-off of the switch 304, the gate terminal has the same potential as the source terminal and the enhancement type N-channel MOSFET 306 is turned off. This is because the current flowing through the switches 304 and 305 is also intercepted by the diode 302.
At the above-described time of charging and at the time of discharging to be described later, the switches 304 and 305 are turned off. Accordingly, when the switches 304 and 305 are turned on at the time of stopping the charging and the switches 304 and 305 are turned on at the time of stopping the discharging as will be described later, two switches are always simultaneously turned on or turned off. As a result, it is not necessary to independently control the switches 304 and 305 and it is possible to simplify the configuration of the control circuit.
A discharging inhibited state will be described below. When a load is connected between the V+ terminal 120 and the V− terminal 121, the voltage Vds between the drain terminal and the source terminal of the enhancement type N-channel MOSFET 306 has a negative value. The control circuit 102 detects that Vds is negative, turns on the switch 301, and turns off the switches 304 and 305. Accordingly, the gate terminal of the enhancement type N-channel MOSFET 306 has a potential higher by the voltage of the secondary battery 101 than the drain terminal and the enhancement type N-channel MOSFET 306 is switched to an electrically-connected state.
When the secondary battery 101 is discharged and the battery voltage reaches a set lower limit value, the control circuit 102 turns off the switch 301 and turns on the switches 304 and 305. Then, the gate terminal of the enhancement type N-channel MOSFET 306 has the same potential as the drain terminal and the enhancement type N-channel MOSFET 306 is switched to the OFF state. As a result, a discharging current is intercepted to prevent the secondary battery 101 from being over-discharged. At this time, a diode 303 is reversely biased to prevent a current from flowing through the switch 304 and the switch 305.
When the discharging current is intercepted, a voltage drop due to internal resistance disappears and the voltage of the secondary battery 101 increases. In order to prevent discharging from being restarted due to the increase in voltage, the discharging inhibited state only has to be maintained until the secondary battery 101 is charged to a certain extent and the voltage thereof becomes equal to or greater than a set value after the discharging is inhibited. When a charging circuit is connected between the V+ terminal 120 and the V− terminal 121 in the discharging inhibited state, Vds is switched from negative to positive. The control circuit 102 only has to control the switches 301, 304, and 305 so as to charge the secondary battery when Vds is positive and to intercept the discharging current when Vds is negative.
In the above description, both switches 304 and 305 are turned on at the time of stopping the discharging. However, even when the switch 305 is turned off, the discharging can be similarly stopped. Since the switch 304 is turned on regardless of the turning-on and turning-off of the switch 305, the gate terminal has the same potential as the drain terminal and the enhancement type N-channel MOSFET 306 is turned off. This is because the current flowing through the switches 304 and 305 is also intercepted by the diode 303.
When both switches 304 and 305 are turned on at the time of stopping the discharging, two switches are always simultaneously turned on or turned off as described above. Accordingly, it is not necessary to independently control the switches 304 and 305 and it is possible to simplify the configuration of the control circuit 102.
The enhancement type N-channel MOSFET 306 has diodes 321 and 322 built therein. However, these diodes are reversely connected in series to each other and thus are not electrically connected, which does not affect the above-mentioned protection operation.
The enhancement type N-channel MOSFET 306 may have a horizontal structure or a vertical structure. When the horizontal structure is employed, the enhancement type N-channel MOSFET 306 and the control circuit 102 can be easily configured as a single IC. Accordingly, since the overcharge and over-discharge protecting circuit including one IC and two switches in the related art can be configured as a single IC, it is possible to achieve a decrease in size and a decrease in cost. On the other hand, when the vertical structure is employed, it is possible to achieve a decrease in loss in comparison with the horizontal structure.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-102182 (FIG. 9)